This invention relates generally to a method and apparatus for mounting a semiconductor wafer on a polishing block to hold the wafer during polishing. More particularly, the invention relates to an infrared heater for heating the polishing block to soften a bonding agent applied to the block so that the wafer may be temporarily bonded to the block.
The faces of semiconductor wafers are polished during manufacture to provide very smooth finish and surface flatness. Semiconductor wafers must be polished so they are extremely flat and smooth before printing electronic circuits on them by electron beam lithography or photolithography. Flatness is critical to maintain resolution of the printed lines which can be as narrow as one micron or less. However, because the wafers are thin, they may flex during the polishing operation if they are not rigidly supported over their entire areas, or the necessary surface flatness and finish may not be achieved. Thus, it is important that the wafers are supported over their entire areas during polishing.
To prepare the wafers for polishing, they are mounted on flat polishing blocks using a bonding agent or wax which firmly grips and rigidly supports the wafers. The bonding agent is applied in a thin coat to one face of the polishing block, and the block and agent are heated so that the agent dries and softens. The wafer is pressed against the softened agent, and, as the agent cools, it firmly grips the wafer so that the wafer cannot easily be pulled or sheared off the block during the polishing operation. Further, the agent stiffens as it cools so that it rigidly supports the wafer during the polishing operation and the needed surface flatnesses and finishes may be obtained.
In the past, an apparatus known as a steam pot was used to heat polishing blocks coated with bonding agents. As its name implies, a steam pot is a vessel containing boiling water. To heat the block, it is seated in an opening at the top of the steam pot so that the side of the block opposite the bonding agent is exposed to the steam inside the vessel. The steam heats the block by convection and conduction heat transfer. The block conducts the heat from its side exposed to the steam to the side to which the bonding agent is applied, thereby drying and softening the bonding agent. However, because the block is cooler than the steam, water condenses on the side exposed to the steam. Although the side of the block having the bonding agent does not face the interior of the steam pot, water will occasionally come in contact with the bonding agent. When water contacts the bonding agent, it becomes pitted which causes small, localized portions of the wafer to be inadequately gripped and supported. As a result, when the polishing block is heated with a steam pot and water condenses or splatters on the block, it must be dried before further processing to avoid water contact with the bonding agent.
Most previously used bonding agents contained trichlorethylene as a solvent. However, because trichlorethylene has been identified as possibly adversely impacting the environment, other bonding agents which do not contain trichlorethylene have been introduced. For instance, water-based bonding agents have been developed. However, these "green" water-based bonding agents do not perform well when heated in steam pots because they are highly water dispersible. Thus, when water droplets come in contact with the agents, large voids occur and the wafers are not adequately supported or gripped.
In an attempt to reduce water splatter, the amount of power delivered to the steam pots has been reduced, but this reduced the steam production which increased the time it took to sufficiently heat the blocks. Longer heating times are undesirable because they increase production time and costs. To speed up production, softer bonding agents have been used so that the blocks did not need to be heated to as high a temperature Thus, lower power settings and shorter heating times could be used. However, the softer agents are stickier, thereby preventing the wafers from being easily released after polishing and substantially offsetting savings in production achieved using the softer bonding agent.